1. Field of the Invention
This invention relates to lamp dimmer circuits for high intensity, gaseous discharge (HID) lamps, and more particularly to a dimmer circuit that supplies a square wave current to the lamp at a constant frequency, the duty cycle of which is both externally controllable to provide a change in lamp intensity and internally regulated in accordance with the amount that the current flowing through the lamp varies from a settable norm.
2. Description of the Prior Art
U.S. Pat. No. 3,816,794, Snyder, describes a circuit employing a two-part reactive ballast connected in series with a high intensity, gaseous discharge lamp. One of the two elements of the ballast is connected across the main terminals of a triac operating as a gated bypass means. When the triac conducts, a current path is established through the triac, at least partially bypassing the reactive element. The duration of conduction determines the total amount of current through the ballast, and hence through the lamp, thereby providing a means for establishing the brightness of the lamp.
In the circuit described in U.S. Pat. No. 3,816,794, low gate source or drive voltage to the gate of the gated bypass triac is derived from a potentiometer, an isolating transformer circuit, a second triac and a Zener diode network, together with other components. The gated bypass triac is fired from a gate source in phase with line voltage, the amplitude being controlled by a gate-signal control device including a Zener diode to properly time the turning on of the triac in relation to lamp current. The Zener diode also prevents the triac from being triggered past a time where there might be opposite polarity ballast-element voltage and lamp current, which would cause flicker of the lamp.
U.S. Pat. No. 3,894,265, Holmes, et al., discloses a circuit that provides a control network for a gated bypass network similar to that shown in the U.S. Pat. No. 3,816,794 patent, the control network including a programmable unijunction transistor. The gating of the bypass triac is by ac gating. Ready connection to single power and three-phase power systems is achieved.
Variations in controlling the timing operations to a gated semiconductor connected for at least partial current bypass operation of a ballast connected to an HID lamp are shown in the following patent applications: Patent application Ser. No. 927,555, "Optocoupler Dimmer Circuit for High Intensity Gaseous Discharge Lamp," filed July 24, 1978, Nuver; patent application Ser. No. 930,913, "High Frequency Dimmer Circuit for High Intensity Gaseous Discharge Lamp Dimmer," filed Aug. 4, 1978, Nuver; patent application Ser. No. 936,883, "Non-Interfering, Overlapping High Frequency Signalling for Lamp Dimming Circuit," filed Aug. 25, 1978, Nuver; and patent application Ser. No. 941,157, "Adjustable DC Pulse Circuit for Variation Over a Predetermined Range Using Two Timer Networks," filed Sept. 11, 1978, Nuver; all commonly assigned to the same assignee as the present application.
Although there are many schemes for gating a semiconductor device for at least partial current bypass of a ballast connected to an HID lamp to which an applied ac source voltage is applied, applicant employs a unique combined switching regulator and transistor bridge arrangement operating at a constant frequency and varying the duty cycle of the current through the lamp at a high frequency rate as controlled by a voltage setting and sensed by a current sensor in series with the lamp to thereby effectively provide a dim/bright current through the lamp.
It is therefore a feature of the present invention to provide an improved dimmer operating to provide a constant frequency, variable duty cycle operation to an HID lamp.
It is another feature of the present invention to provide an improved dimmer operating to provide a constant frequency, variable duty cycle operation to an HID lamp at a high frequency rate while reversing the current therethrough at a low frequency rate.
It is still another feature of the present invention to provide an improved dimmer having all of the desirable features set forth above.